32 PLANORBIS ARMIGERUS. 



OBSERVATIONS. 



The teeth within the aperture may be con- 

 sidered analogous to those of certain Hehces, 

 but they differ in the important point of being 

 present at every stage of growth. They are 

 present when the shell is less than a line in 

 length, and as but one set exists in full grown 

 individuals, we must infer that they are absorbed 

 and reproduced from time to time. In over- 

 grown specimens, like those figured, it some- 

 times happens that the teeth are wanting; as if, 

 after their absorption, the energies of the ani- 

 mal were too far exhausted to reproduce them. 

 The outer ones seem to be formed successively 

 from left to right; the small one on the right 

 appearing last, and in its absence, the shell has 

 been described by Say and Gould as being but 

 five-dentate. 



In Segmentina, three large teeth project, so 

 as to leave a triradiated aperture for the pass- 

 age of the Mollusk; and as the earlier ones are 

 not absorbed, several of the partitions thus 

 formed, are always present. 



